Inventory of irrigation reuse (or wetland concentration) pits in the Rainwater Basin Wetland Complex. The inventory is updated annually removing pits that have been filled through watershed or wetland restorations by Rainwater Basin Joint Venture conservation partners. Irrigation reuse pits are remnant structures associated with gravity irrigation. Center pivot irrigation has largely replaced gravity irrigation in the region but, the irrigation reuse pits that remain in the watersheds of Rainwater Basin wetlands intercept rainfall that would otherwise runoff to the wetland. Concentration pits are structurally similar to irrigation reuse pits but were constructed within the wetland to concentrate water and open portions of the wetland to other uses, such as crop production. Historically, some pits may have function as both a irrigation reuse pit and concentration pit.
Nearly all irrigation re-use pits in the RWB are rectangular. Pits are generally assumed to have a 2.5:1 slope for all sides and an average depth of 8 ft, making the bottom of the pit 20 ft smaller in all dimensions. The following formula was developed by NRCS engineers to estimate pit volume in cubic feet.:
(A + 4C + B) x (D / 6)
where A is the surface area of the top of the pit as delineated in GIS (ft2 ), C is the surface area of the bottom of the pit (ft2 ), B is the surface area halfway down the pit (ft2 ), and D is the average depth (8 ft). C was calculated as 20 ft smaller in all dimensions than the top surface area. B was determined as the mean of A and C. Cubic feet were then converted to acre-feet.
